Start the recipe by washing and hulling the strawberries. Place in a large glass bowl and sprinkle over the lemon juice and sugar. Mix well and cover with a clean tea towel. Leave the fruit overnight to dissolve the sugar and bring out the juices. The next day, tip the strawberries into a large clean saucepan, and heat slowly to dissolve the sugar. Carry on heating till boiling point, and then continue to boil vigorously for 4 minutes. Add the knob of butter and mix well. Remove the scum from the surface of the jam with a ladle. Pour the jam into 3 sterilized preserving jars, and immediately cover each jar with a piece of waxed paper, before closing up the jars. Store the jam in a cool place. The jam should stay fresh for up to 6 months, but once opened store in the fridge.